Capsules for accommodating pharmaceutical preparations and functional foods are generally prepared using gelatin and hydroxypropyl methylcellulose (HPMC) as base materials.
A gelatin capsule may lead to improved productivity and price competitiveness. However, if the gelatin capsule contains moisture of 10 wt % or less, plasticity and impact resistance thereof may be reduced. In addition, since the use of gelatin is limited due to mad cow disease or the like, an HPMC capsule formed of a plant-based material without using gelatin has been widely used.
In general, methods of preparing hard capsules are classified into two types, freeze-gelation and thermal gelation, according to gelation properties.
A capsule may be prepared using freeze-gelation including: heating a gelatin solution that is gelated at room temperature or a HPMC solution including carrageenan, agar, sodium alginate, gellan gum, and/or pectin which are gelated at room temperature and maintaining the solution at a high temperature to mature the gelatin or the HPMC of the solution; immersing a cold mold pin in the solution and coating the mold pin with a predetermined amount of the solution; and taking the mold pin out of the solution, immediately applying cold air having a temperature of about 20° C. to the mold pin to gelate the gelatin or the HPMC of the solution, and drying the gel. In the freeze-gelation, carrageenan, agar, sodium alginate, gellan gum, pectin, or the like are widely used in the preparation of capsules as a gelating agent since they bind to metal ions such as potassium, calcium, and sodium to increase gelation capability. However, when a capsule including impurities such as carrageenan is orally administered, the impurities react with metal salts in the gastric juice or intestinal juice, so that a binding force among components of the capsule increases, thereby inhibiting disintegration of the capsule.
A capsule may also be prepared using thermal gelation using gelating features of HPMC in an HPMC solution at a high temperature. A high-temperature mold pin is immersed in an HPMC solution maintained at a temperature equal to or greater than room temperature to thermally gelate the HPMC of the HPMC solution coated on the mold pin by the heat of the mold pin.
However, since the capsule is orally administered, disintegrated by gastric juice and absorbed in the body, it may not be used in case that main ingredients and excipients of pharmaceutical preparations and functional foods filled in the capsule are instable in an acid, irritate the stomach, or generate an odor that can be regurgitated. In this case, the surface of the capsule is coated with an enteric base material after filling the capsule, so that the capsule can have enteric properties.
However, for the coating of the capsule with the enteric base material, a process for the coating is further required, and thus the manufacturing costs increase. Furthermore, an organic solvent contained in a coating solution may remain on the surface of the capsule during the coating, an identification code of the capsule may become invisible due to the coating, or the quality of appearance of the capsule may be deteriorated after the coating.
Therefore, much research has been conducted to develop an enteric hard capsule. However, an enteric hard capsule with high quality and high productivity has not been commercialized.